This invention relates to an ultrasonic bonding tool apparatus and method of bonding a conductor to different conductive bonding sites useful, for example, to bond gold wires or ribbons to different bonding sites.
Conductive gold wires and ribbons are often used to electrically connect the bond pads of an electronic component to the interconnect leads of a support package or substrate.
It is known in the art to use both high temperatures and/or ultrasonic energy in automated wiring equipment to bond the gold conductors to various bond sites.
Some electronic components, however, cannot withstand the high temperatures associated with prior art bonding techniques. In addition, cooling systems must be employed with the ultrasonic wiring equipment to prevent damage thereto and/or to prevent variations in the ultrasonic energy delivered to the bonding tool.
Gold can be bonded at room temperature using ultrasonic energy provided, however, that the gold conductor is very clean. Gold can be cleaned by methods such as plasma cleaning and then reliably bonded at much lower temperatures than gold surfaces which have been exposed to airborne contaminants for even a few minutes. Plasma cleaning gold wire and ribbon, however, results in an additional cost to the finished assembly and, because the bondability of clean gold surfaces degrades rapidly on exposure to air, conventional bonding techniques require that the bonds be made within a short time after cleaning. If the specified time period lapses before bonding is completed, the material must be re-cleaned.
In U.S. Pat. No. 5,894,983 incorporated herein by this reference, a support structure is heated to only 25-85xc2x0 C. using a convection heat source and the ultrasonic bonding tool is vibrated at a higher frequency, (e.g., 122 KHz-140 KHz).
This patent exemplifies the tradeoff between temperature and bond strength. In general, unless the gold is very clean, a good ultrasonic bond at low temperatures is difficult to reliably obtain. Bonding engineers are well aware that gold to gold bonding is not difficult as long as there is no temperature restriction imposed on the process. In the semi-conductor industry, bonding is routinely performed at 300xc2x0 C. or higher. Larger and more complex assemblies, however, are usually fabricated from materials that degrade at elevated temperatures. Thus, for many projects, the bonding engineer is requested to select a room temperature process. The only known room temperature process available is ultrasonic bonding. Gold, however, is difficult to ultrasonically bond at room temperature unless, as stated above, it is very clean. Thus, under normal conditions, gold must be ultrasonically bonded at elevated temperatures. The temperatures selected by the bonding engineer is generally as high as the process specifications will permit. Higher temperatures yield stronger bonds and smaller bond strength deviation.
Thermosonic bonding is a combination of thermo-compression and ultrasonic bonding in which heat is intentionally added to the vibrating bond tool in a steady state manner. In addition, heat is often added to the substrate. Heating of the tool or substrate, however, can create problems if the materials being bonded are subject to thermal degradation. Modem thermosonic bonding usually employ a heated tool and a substrate temperature of only 125xc2x0 C. or less. Thus, there is always a trade off between bond temperature, bond strength, and reliability.
It is therefore an object of this invention to provide a thermosonic bonding apparatus, a bonding tool, and a method which achieves higher reliability bonds.
It is a further object of this invention to eliminate the need for and the expense associated with an intensive gold cleaning processes.
It is a further object of this invention to eliminate the need for heat sources.
It is a further object of this invention to prevent thermal damage to electronic components during the bonding process.
It is a further object of this invention to provide a thermosonic bonding tool which has a very long useful life.
This invention results from the realization that by locally heating only the tip of a bonding tool with a short thermal impulse synchronized with the delivery of ultrasonic energy to the bonding tool, the need for ultra clean gold and the need for heat sources is eliminated without causing thermal damage to the electronic components and yet, at the same time, a better bond is made because both heat and ultrasonic energy are used.
This invention features a thermosonic bonding apparatus comprising an ultrasonic transducer; a bonding tool including a high resistivity tip and a low resistivity shaft extending from the tip; a tool support arm interconnecting the bonding tool and the ultrasonic transducer to vibrate the high resistivity bonding tool tip; and a voltage source connected to the bonding tool to heat the high resistivity bonding tool tip.
In one example, the bonding tool includes an insulative gap in the shaft to direct current to the high resistivity tip. Typically, the voltage source is connected to the bonding tool by a pair of conductors each contacting the shaft on opposite sides of the insulative gap. Preferably, the conductors contact the shaft at a nodal location.
In the prototype example, the bonding tool was made of a ceramic alloy composition and the high resistivity tip was wedge shaped.
A thermosonic bonding tool apparatus in accordance with this invention includes a bonding tool including with a high resistivity tip, and a low resistivity shaft extending from the tip; means for imparting vibrations to the high resistivity tip; and means for locally heating the high resistivity tip. The means for imparting vibration typically includes an ultrasonic transducer and a tool support arm interconnecting the bonding tool and the ultrasonic transducer to vibrate the high resistivity bonding tool tip. The means for locally heating may include a voltage source connected to the bonding tool to provide pulses of voltage to the bonding tool to heat the tip of the bonding tool in a pulsed fashion. Alternatively, other heating sources may be used such as a laser. The bonding tool may include an insulative gap in the shaft to direct current to the high resistivity tip and the voltage source is connected to the bonding tool by a pair of conductors each contacting the shaft on opposite sides of the insulative gap. Preferably, the conductors contact the shaft at a nodal location. The entire bonding tool may be made of a ceramic alloy composition or, alternatively, just the tip is made of a ceramic alloy and the shaft is then made of a different low resistivity material.
The method of bonding a conductor to conductive bonding sites in accordance with this invention includes the steps of a) bringing the conductor into contact with a first bonding site via the tip of a bonding tool; b) applying a thermal pulse to the tip of the bonding tool; c) vibrating the tip of the bonding tool while it is heated to thermosonically bond the conductor to the first bonding site; d) terminating the thermal pulse and the vibrations; e) bringing the conductor into contact with a second bonding site via the tip of the bonding tool; and f) repeating steps b) and c). The application of the thermal pulse may be automatically synchronized with the application of the ultrasonic energy.
The tip of the tool can be heated to between 50xc2x0 C. and 1,000xc2x0 C. and vibrated at between 40 KHz-80 KHz. Typically, the bonding tool used is configured as described above but this is not a necessary limitation of the subject invention.
In one embodiment, the bonding tool includes a high resistivity ceramic tip, and a low resistivity shaft extending from the ceramic tip. An ultrasonic transducer is employed to vibrate the high resistivity ceramic tip which is also locally heated in a pulsed mode by a source such as a voltage source.